Daichi Miyahara attends Suimei University of the Arts High School alongside classmates Sorata Kanda and Nanami Aoyama. He previously shared a room with Sorata in the general dormitory prior to Sorata's relocation to Sakura Hall. Their friendship persists, with Miyahara notably aiding Sorata in concealing stray cats in violation of dormitory rules.

Miyahara possesses a tall, slim yet muscular physique that conveys a strong presence. His personality blends cheerful, outgoing energy with off-beat comedic tendencies, balanced by sincere care for others and a readiness to assist. An alternative perspective portrays him as organized, dependable, private, and rule-abiding, though his reserved nature occasionally sparks misunderstandings. He actively participates in the swimming club and co-founds the Film Club with friends Chihiro and Makoto, dedicating efforts to creating short films. Additionally, he serves on the arts university's cultural festival executive committee.

Miyahara develops romantic feelings for Nanami Aoyama, culminating in a confession during the cultural festival in an empty classroom. Despite her rejection, he maintains an honest, supportive friendship with her devoid of resentment.

His surname Miyahara combines the characters for "temple/shrine/palace" (宮) and "field/plain" (原), while his given name Daichi translates to "earth/solid earth/ground" (大地).